Method of improving solderability of tin plate



3,281,341 METHQD F EMPRUVENG SOLDERABILITY 015 TM PLATE lssa .l. Kharouf, Penn ll-lills Township, Allegheny County, and Richard A. Neish, Snowden Township, Allegheny County, Pa, assignors to United States Steel Corporation, a corporation of Delaware No Drawing. Filed Apr. 18, 1963, Ser. No. 273,847 1 Claim. (Cl. 204-29) This invention relates to a method of treating tin plate to improve its solderability.

Tin plate has been made by various methods but, regardless of the particular method employed, has been characterized by highly variable solder-ability. Tin plate having light coating Weights of tin, such as 0.25 pound of tin per base box (31,320 square inches of sheet or 62,640 square inches of surface) is particularly likely to have poor solderability. This property is evaluated by means of a capillary rise test which indicates whether molten solder will completely fill the side seam of a can. In the test, a 1-inch by 3-inch panel of tin plate is folded lengthwise and then vertically inserted into a bath of molten solder to a predetermined depth. The height to which the molten solder rises above the level of the bath within the fold of tin plate is measured. The higher the capillary-rise values, the better the soldering performance of the tin plate and vice versa.

The primary object of our invention is to provide a method of producing tin plate having improved solderability. Briefly stated, our invention comprises treating the tin plate during or immediately prior to the conventional passivation treatment, with a water solution of a non-ionic organic wetting agent, more specifically, an alkyl aryl ether of polyalkylene glycol such as the reaction product of t-octyl, n-nonyl or dodecyl alcohol with ethylene oxide. This product is:

where R is a saturated alkyl hydrocarbon radical containing 8 or more carbon atoms and x is an integer greater than 8. More particularly, R is a saturated alkyl hydrocarbon radical selected from the group t-octyl, n-nonyl and dodecyl. Emulsifiers or wetting agents of the type designated are obtainable commercially, i.e., Tergitol wetting agent or Triton wetting agent.

A complete understanding of our invention may be obtained from the following detailed description and explanation of a typical example of the method.

In the normal practice of tin plate manufacture, it is conventional to passivate the tin surface. Passivation may be effected by the method of Mason Patent No. 1,827,204 or Neish Patent No. 2,606,866. According to our invention, we modify the passivating solution of either of these patents by adding thereto a non-ionic type Wetting agent such as the aforementioned alkyl aryl ether of a polyalkylene glycol, in an amount of from 0.005 to 0.02 percent of wetting agent by volume of passivating solution (dilute aqueous sodium-dichromate). Some Wetting agents which are particularly effective are Triton X100 and Tergitol NP-27, NPX or XD. The passivation is then effected in the usual way, for example, by cathodic electrolysis to the extent of about 30 to 50 coulombs per square foot of tin plate surface, while maintaining the solution at about 130 F. The preferred concentration range for the added wetting agent is from 0.01 to 0.02 percent by volume.

nited States Patent 0 3,281,34l Patented Oct. 25, 1966 The invention may also be practiced by rinsing the tin plate before passivation, in a water solution of Wetting agent in the same range of concentrations.

Our method materially improves the solderabi'lity of the tin plate as evidenced by an increase of from 60 to 250 or 300 percent in the capillary rise exhibited by the product thereof, as compared with the product passivated in the normal Way, i.e., without the addition of a wetting agent. The fact that the solderability of tin plate is significantly improved by the use of our invention may be explainable on the basis of the following theory.

Since unpassivated tin plate exhibits satisfactory solderability, it was apparent that poor solderability is associated with the passivating treatment, and, more specifically, with the passivation of the steel or alloy surfaces existing at small pores which are always present in tin plate having only a light coating of tin. Solderability tests made on tin plate having heavy, relatively porefree tin coatings, however, showed that passivation of the tin itself did not adversely affect solderability. It follows therefore that the wetting agents shown by us to be effective must be adsorbed selectively on the exposed steel or alloy surfaces, thereby preventing passivation of these surfaces without interfering with the passivation of the tin itself. This, of course, is contrary to what would normally be expected, since such compounds are usually added to solutions to decrease the surface tension and, therefore, enhance any reaction which occurs between the metal surface and the treating solution. We believe that, because of the selective adsorption of these compounds, the tin surface can be passivated in the usual manner without adversely affecting the soldera-bility of the tin plate product. In fact, the solderability is materially improved.

Our invention has the advantage that, although the solderability is increased materially, other desirable properties such as resistance to corrosion under humid conditions and to discoloration on baking and the lacquerability are not impaired.

Although we have disclosed herein the preferred embodiment of our invention, We intend to cover as well any change or modification therein which may be made without departing from the spirit and scope of the invention.

We claim:

A method of making tin plate of improved solderability which comprises electrolytically passivating the plate in a dilute aqueous sodium dichromatesolution and subjecting it, not later than said passivation to contact with a water solution containing from 0.005 to 0.02 percent by volume of an alkyl aryl ether of polya'lkylene glycol produced by the reaction of ethylene oxide and an alcohol selected from the group consisting of t-octyl, n-nonyl and dodecyl alcohol.

References Cited by the Examiner UNITED STATES PATENTS 1,970,578 8/ 1934 Schoeller et al.

2,420,602 5/1947 Kingerley 204145 2,601,863 7/1952 Murphy 13440 X 2,606,866 8/1952 Neish 204- X 2,916,459 12/ 1959 Arnold et al.

2,987,426 6/1961 Shaw 134-29 3,145,180 8/1964 Dupre et al.

JOHN H. MACK, Primary Examiner. R. MIHALEK, Assistant Examiner. 

